Two identical cars with mass m = 1,325 kg are traveling in opposite directions at speeds of 54 m/s. they collide head on. after the collision, both cars recoil and move at 43 m/s, in opposite directions. according to the law of conservation of linear momentum, what type of collision occurred and how much kinetic energy was lost? a) the collision was inelastic and 1.4 × 106 joules of energy were lost. b) the collision was inelastic and 0.7 × 106 j joules of energy were lost. c) the collision was perfectly elastic and 1.4 × 106 joules of energy were lost. d) the collision was perfectly elastic and 0.7 × 106 joules of energy were lost.

In a collision the kinetic energy is sometimes the same before the collision as it is afterwards - such a collision is called an elastic collision. Perfectly elastic collisions are rare - usually some of the kinetic energy is converted to sound or heat. Even a trampolinist colliding with a trampoline bed loses energy as the bed stretches and heats up. A perfectly inelastic collision is one where all the kinetic energy is converted into other forms - a piece of soft putty falling onto the floor is an almost perfectly inelastic collision, the kinetic energy of the moving putty is converted into heat and a little sound as the putty is deformed on impact. Most real collisions are usually somewhere between the two extremes. 

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